Underground boring technology is well-known and commonly used for a variety of applications. It is an accepted and frequently used method for installing underground utilities under roadways, rail beds, lawns, fences, and other surface obstructions. Underground boring is a desirable alternative to digging an open trench, particularly in developed areas.
A variety of boring tools are available, including rotary flexible rod devices, auger devices, pipe pushers, and air or hydraulic powered impact type earth boring tools (also known as percussive moles). Directional boring systems also have been developed which let the operator control and monitor the path of an underground boring tool.
A conventional boring tool assembly includes a drilling frame or rig, sections of drill pipe or flexible hose, and a boring tool. The boring tool may include a steering assembly and tracking instrumentation. Hydraulic power generally is used to control various functions of the drilling frame, while compressed air generally is used to operate the boring tool. Typically, compressed air is supplied through the drill pipe or by a flexible hose. The boring tool may be a drilling motor driving a cutting bit to drill through rock or a percussive mole for compacting a bore hole in soil. The present invention has particular applicability to pneumatically powered percussive moles in which compressed air is provided to the percussive by the drill pipe rather than a flexible hose.
As the percussive mole progresses away from the drilling frame, additional sections of drill pipe are added between the boring tool and the drill frame. The successive lengths of drill pipe are generally referred to as comprising a drill string. As each section of drill pipe is added to the drill string, air flow to the tool must be interrupted and the drill string emptied of air pressure. This is normally accomplished by a valve on the drilling frame which opens and closes the supply of compressed air. Once the connection of another section of drill pipe is completed, the air flow to the tool can be initiated by opening the compressed air valve on the drilling frame.
The increasing length of the drill string is, in effect, forming an increasingly long expansion chamber. Expansion of the compressed air entering the empty drill string drastically reduces the initial air pressure and energy potential available to start the tool in operation at the end of the drill string. This energy potential builds up slowly because of a limited or fixed capacity for generating compressed air entering the lengthened drill pipe. Since most job site air compressors have small air tanks, the time required to fill the pipe increases as the length and/or diameter of pipe increases. If pressure build-up inside the tool is slow, pressure leaks in the system preclude reaching start-up pressures and the tool will not start.
Percussive moles, in particular, require a certain surge of energy to initiate operation because of hammer inertia, internal friction, and leakage. This may be further aggravated by ineffective lubrication or frost conditions from air expansion within the tool. In cold atmospheric conditions, a percussion mole may freeze moisture in the tight seal areas. Accordingly, in order to overcome the difficulties of starting a percussive mole at the end of an increasingly long drill string, it is desireable to provide a high energy starting pulse of compressed air to the tool.
In a pending, related application entitled "Earth Boring Apparatus with Control Valve", Serial No. 136,401, filed December 22, 1988, now Patent No. 4,834,193, issued May 30, 1989, there is disclosed a downhole pressure operated valve assembly designed to provide a high energy starting pulse of compressed air to a downhole tool. The valve is positioned "down the hole," that is, adjacent the tool. The valve disclosed in U.S. patent application Ser. No. 4,834,193 is operable in response to the pressure of the working fluid, i.e., compressed air, in a drill string or flexible conduit. As the flow of working fluid is supplied to the downhole tool, the downhole valve disclosed in U.S. Pat. No. 4,834,193 is kept closed until a predetermined pressure is reached, at which time the valve is opened and a pulse of working fluid is supplied to the downhole tool. The valve is kept open until the pressure drops to a second predetermined pressure, substantially lower than the first predetermined opening pressure, at which time the valve in U.S. Pat. No. 4,834,193 is closed. This valve, however, requires that the opening and closing pressures be predetermined. This does not provide the flexibility to vary the pressures at which the valve is opened or closed to thereby accommodate a variety of operating conditions.
Articles on page 18 in the Autumn 1986 issue of MICROTUNNELING magazine and on page 18 of the July 1986 issue of UNDERGROUND magazine mention a percussive hammer having an electrically operated downhole solenoid valve connected to the hammer. This solenoid valve is manually operated to provide the kick or boost required to get the percussive equipment moving properly. However, this valve requires a separate supply of electric power and an additional supply line in the bore hole to provide electric power to the downhole valve.